The adjtime() function gradually adjusts the system clock (as
returned by gettimeofday(2)). The amount of time by which the clock
is to be adjusted is specified in the structure pointed to by delta.
This structure has the following form:
struct timeval {
time_t tv_sec; /* seconds */
suseconds_t tv_usec; /* microseconds */
};
If the adjustment in delta is positive, then the system clock is
speeded up by some small percentage (i.e., by adding a small amount
of time to the clock value in each second) until the adjustment has
been completed. If the adjustment in delta is negative, then the
clock is slowed down in a similar fashion.
If a clock adjustment from an earlier adjtime() call is already in
progress at the time of a later adjtime() call, and delta is not NULL
for the later call, then the earlier adjustment is stopped, but any
already completed part of that adjustment is not undone.
If olddelta is not NULL, then the buffer that it points to is used to
return the amount of time remaining from any previous adjustment that
has not yet been completed.

The adjustment that adjtime() makes to the clock is carried out in
such a manner that the clock is always monotonically increasing.
Using adjtime() to adjust the time prevents the problems that can be
caused for certain applications (e.g., make(1)) by abrupt positive or
negative jumps in the system time.
adjtime() is intended to be used to make small adjustments to the
system time. Most systems impose a limit on the adjustment that can
be specified in delta. In the glibc implementation, delta must be
less than or equal to (INT_MAX / 1000000 - 2) and greater than or
equal to (INT_MIN / 1000000 + 2) (respectively 2145 and -2145 seconds
on i386).

A longstanding bug meant that if delta was specified as NULL, no
valid information about the outstanding clock adjustment was returned
in olddelta. (In this circumstance, adjtime() should return the
outstanding clock adjustment, without changing it.) This bug is
fixed on systems with glibc 2.8 or later and Linux kernel 2.6.26 or
later.

This page is part of release 4.13 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2017-09-15 ADJTIME(3)